1. Field
Apparatuses and methods consistent with the exemplary embodiments relate to an electrically-heated catalytic converter disposed in an exhaust system for exhaust gas.
2. Description of Related Art
In various industrial fields, various efforts are underway on a worldwide scale to reduce environmental impacts and environmental loads. In the field of the automobile industry, the widespread use of not only gasoline engine vehicles with high fuel efficiency performance but also so-called eco-friendly vehicles, such as hybrid vehicles and electric automobiles, has been promoted, and the development focused on further enhancement of the performance of such vehicles has been advanced.
An exhaust system for exhaust gas, which is configured to connect a vehicle engine to a muffler, may be provided with an electrically-heated catalytic converter (EHC) configured to clean exhaust gas under normal temperatures, and, in addition, to clean exhaust gas by activating a catalyst as quickly as possible through electric heating under cold environment. The electrically-heated catalytic converter has a configuration in which, for example, a pair of electrodes is attached to a honeycomb catalyst disposed in the exhaust system for exhaust gas and these electrodes are connected to each other via an external circuit provided with an electric power source. In the electrically-heated catalytic converter, the honeycomb catalyst is heated by supplying electric current to the electrode to increase the activity of the honeycomb catalyst. In this way, the electrically-heated catalytic converter removes toxic substance in exhaust gas passing therethrough.
The electrically-heated catalytic converter typically includes an outer pipe (metal case), a substrate having a honeycomb structure and a heat-generating property, an insulating mat (holding member), a pair of electrodes, and an external circuit. The outer pipe has a hollow space. The substrate is disposed in the hollow space of the outer pipe, and has catalyst coated layers. The insulating mat is interposed between the outer pipe and the substrate. Each of the electrodes is attached to a surface of the substrate, in a region where the mat is not disposed. The external circuit connects the electrodes to each other. A honeycomb structural body having such a configuration is described in Japanese Patent Application Publication No. 2013-198887 (JP 2013-198887 A).
Specifically, electrode films for diffusing electric currents throughout the substrate as evenly as possible are disposed at portions of the surface of the substrate where the electrodes are disposed. The electrically-heated catalytic converter has a configuration in which electrode terminals are attached to the surface of the substrate through openings provided in the electrode films. External electrodes (lead terminals) are attached to the electrode terminals via brazing filler metals, and a cable leading to the electric power source constitutes the external circuit. Alternatively, the electrode terminals may be attached to the surfaces of the electrode films without providing openings in the electrode films.
As described above, the components of the electrically-heated catalytic converter include the substrate supporting catalysts and having a honeycomb structure, the electrode films disposed on the surface of the substrate, the electrode terminals disposed on the surfaces of the electrode films, and the external circuit. Examples of the substrate are roughly classified into metal substrates and ceramic substrates. It is a known fact that it is difficult to use metal substrates in hybrid vehicles (HV) and plug-in hybrid vehicles (PHV) because the resistance is too low. For this reason, electrically-heated catalytic converters including ceramic substrates, which are usable in the eco-friendly vehicles, are becoming the mainstream.
The electrode films, the electrode terminals, and the substrate having a honeycomb structure, which constitute the electrically-heated catalytic converter, are required to have the following actions and functions.
Each electrode film is required to have a function as a current collector, and it is thus desirable that each electrode film have a lower volume resistivity than that of the substrate. Further, each electrode film is required to have an electric current diffusion function with which even supply of electric current throughout the substrate is promoted, and it is thus desirable that each electrode film be configured to adjust the flow of electric current so as to diffuse the electric current throughout the substrate as evenly as possible. Each electrode film is attached to the surface of the substrate, and it is thus desirable that a joint interface at which the electrode film is attached to the substrate have a strength against thermal stress, which is equal to or higher than that of the substrate. For this reason, it is desirable that the strength of connection between each electrode film and the substrate be high, and in addition, the thermal expansion coefficient of the substrate and that of each electrode film be approximately equal to each other to minimize the difference in thermal deformation between each electrode film and the substrate. Furthermore, in view of resistance to thermal shock, it is desirable that each electrode film have a thermal conductivity equal to or higher than that of the substrate. From the viewpoint of maintaining the reliability for environment resistance, it is desirable that variations in the volume resistance of each electrode film under the high-temperature oxidation atmosphere be small. The above-described actions and functions required of each electrode film are also required of the electrode terminals.
It is desirable that the substrate, which is a heating element having a honeycomb structure, be configured such that the resistance value can be controlled to the optimal resistance value depending on the intended use, and the current and voltage applied to the substrate. Further, it is desirable that the temperature dependence of the resistance be low and variations in the resistance value be small within a temperature range from −30° C. to 1000° C., which is a usage temperature range of the catalyst. Further, it is desirable that the substrate have high oxidation resistance and high thermal shock resistance, and, in addition, the substrate be configured so as to be easily joined to the electrode films and the electrode terminals.
In the electrically-heated catalytic converter in the related art in which the substrate provided with the electrode films, the electrode terminals, and the external electrodes are secured to each other as described above, the generated thermal stress varies among these components due to a difference in thermal expansion coefficient among these components. For example, the thermal expansion coefficient of a SiC/Si-based substrate is 4×10−6/° C. to 5×10−6/° C., the thermal expansion coefficient of a SiC/Si-based or MoSi2-based electrode terminal is 4×10−6/° C. to 6×10−6/° C., the thermal expansion coefficient of a Ni-based brazing filler metal is 14×10−6/° C. to 15×10−6/° C., and the thermal expansion coefficient of an external electrode made of 20Cr-5Al Steel-Use-Stainless is 11×10−6/° C. to 12×10−6/° C. Hereinafter, 20Cr-5Al Steel-Use-Stainless will be referred to as SUS (20Cr-5Al) where appropriate. Similarly, other kinds of Steel-Use-Stainless will be denoted by their abbreviated names where appropriate. Note that SUS is a standard of stainless steel that is defined by Japanese Industrial Standards (JIS).
There is a significant difference in thermal expansion coefficient particularly between the electrode terminal and the brazing filler metal. This causes a possibility that damages, such as a crack, will occur at an interface between the electrode terminal and the brazing filler metal due to a difference in thermal stress between the electrode terminal and the brazing filler metal. Damages to the interface result in a decrease in the reliability for the electric connection performance.